Well - what I did was to start off by thinking that half inch is not very big, so I got 5 gallons of Prestone (I figured I have some left over!) and then mixed the first gallon in a bucket half and half with water - yielding 2 gallons total of fluid.

Then, using a drill-powered pump I pumped the first line full. When it started coming out the other end, I let it run back into the bucket and kept it going for another couple of minutes. Then I capped both ends of that loop, and moved on to the next one. But the bucket was nearly empty, so I dumped in another gallon of Prestone and another gallon of water, hooked up the drill pump and.....

After a while I went back to the store and got another 7 gallons of Prestone and kept up the same procedure.

Out of 24 gallons total of fluid, I have about a half gallon left, all the lines are full, and I have some 2600 feet of line in place.

Then I crimp-sealed the (return) ends of the lines, crimp-sealed (with an adapter) and air-compressor end to a 3 foot piece of pex and used a "shark-bite" connector to join the free end of the 3-fot piece to the still uncrimped end of my floor line. Hooked 125 PSI to it, left it for 10 minutes and checked for leaks. No leaks. Release the sharkbite from the end of the floo line and re-seal that. Hook the sharkbite to the next loop and repeat the 125 PSI test.

Repeat for all 13 loops.

Yes, there may be some air remaining in the lines, but the air/water separator that will go between the return manifolds and the heater will trap that, and allow for fluids to be topped up